The objective of this research is to determine the molecular structure of ubiquitin, using protein X-ray crystallographic techniques. This small protein has been sequenced and has a molecular weight of 8,565 daltons. Although its exact role in the cell is unknown, it has been implicated in lymphocyte differentiation, regulation of transcription, and as a signal for protease degradation of proteins. The latter two functions involve ubiquitin-protein conjugates. Heavy-atom derivatives of ubiquitin have been prepared, and an electron density map has been calculated. Efforts are now underway to trace the polypeptide chain. The ultimate goal is the high-resolution (1.2-1.5 Angstrom) crystal structure; this structure may help explain the diverse functions of ubiquitin.